1. Field of the Invention
The present invention relates to data communications systems and, in particular, to a line interface device that is switchable between single-ended and differential operating modes.
2. Discussion of the Prior Art
The increasing use of integrated logic circuits, such as TTL, MOS and CMOS components, in computer systems dictates the need for interface circuits. An interface circuit amplifies the low voltage/low current levels at the computer output ports either for transmission over a transmission medium (e.g., twisted pair, coaxial cable) or for use by higher voltage/higher current peripheral devices (e.g., displays, printers).
A line interface circuit is a type of interface circuit that transfers digital information from a computer or peripheral device over a transmission line.
A bi-directional line interface circuit, commonly referred to as a transceiver (TRANSmitter/reCEIVER), typically includes a line driver stage and receiver stage. The line driver amplifies digital signal outputs from the computer system so that they can be properly transmitted on the transmission medium. Conventional line drivers usually include level shifting capability to provide compatibility with different integrated circuit technologies (e.g. TTL) that might be used in the computer's internal logic. The receiver is typically a differential amplifier that receives signals from a two-wire transmission channel and provides a single-ended output representative of the digital information received on that channel.
Referring to FIG. 1, as stated above, a conventional differential transceiver 10 typically combines a differential line driver 12 and a differential receiver 14. Whether the transceiver network 10 operates as a receiver or a line driver (transmitter) is determined by the state of the enable signal DE/RE. In the receiver mode, (DE/RE=0) the driver 12 is disabled by placing its differential outputs in the high impedance state; the differential inputs from the transmission medium are thus applied to the inverting and non-inverting inputs of the differential-to single ended receiver 14 which, in turn, provides a system input via the I/O line. Similarly, in the transmit mode, (DE/RE=1) receiver 14 is disabled by placing its output in the high impedance state; a system output is thus applied to driver 12 via the I/O line which, in turn, provides its differential outputs DO+/RI+ and DO-/RI- to the transmission medium.
In a single-ended configuration, the transceiver's line driver and receiver are single input-single output circuits.
Line interface circuits may be designed for general-purpose applications or may be designed for a specific, industry standard data-communications configuration.
One such industry standard data-transmission configuration is the so-called Small Computer System Interface (SCSI) standard. American National Standard (ANS) X3.131 defines one line interface configuration for single-ended operation and a second line interface configuration for differential operation.
A problem arises because the SCSI single-ended and differential line interface configurations are incompatible. That is, a "single-ended" SCSI computer station cannot be connected to an SCSI system that is configured as "differential".
It would, therefore, be highly desirable to have available a transceiver interface device with the flexibility to be utilized both in single-ended and differential SCSI systems.